Introduction:Exercise places a major challenge on the body due to the increased energy needs and physiological demands put upon the body's nervous, muscular, cardiovascular, metabolic and respiratory systems. As a body goes through these responses and subsequent adaptations to exercise, there are a number of complex hormonal interactions occurring simultaneously. For example, hormones can increase blood pressure, stimulate protein synthesis, and increase the body's metabolic rate. With resistance training variables, several hormonal responses and consequent adaptations which have been observed will be reviewed in this article.

Hormones '101': The BasicsHormones, secreted by endocrine (hormone secreting) glands in the body, are substances that regulate the function of body cells, tissues, organs or systems. Hormones are released from a number of 'traditional' glands, such as the pituitary, testes, ovaries, pancreas, thyroid and the adrenal cortex. More recently, science has also documented hormone secretion from 'non-traditional' sites such as the heart, kidney, liver, and adipose tissue. In reference to gender, the major differences in male and female endocrinology (the study of the hormone secreting glands) come down to the differing reproductive structures (testes versus ovaries). Males produce high levels of testosterone whereas females have higher levels of estrogen and progesterone, and lower levels of testosterone when compared to males.

Hormone Function in Resistance Exercise: Energy ProductionWith resistance exercise there is an immediate increase in epinephrine and norepinephrine (Kraemer and Ratamess, 2005). These hormones increase blood glucose and are important for increasing force production, muscle contraction rate, and energy production (i.e., the synthesis of ATP-the energy currency of cells). These hormones actually begin to rise prior to the resistance training workout (Kraemer and Ratamess, 2005). This is an anticipatory response of the body preparing for the challenging exercise to follow. Interestingly, the elevated blood glucose levels do not typically lead to an increase in insulin unless protein/carbohydrate supplementation precedes the workout (Kraemer and Ratamess, 2005). The increased uptake of blood glucose by the skeletal muscle is occurring due to the increase in function of the cell's glucose transporters, which increase glucose uptake and thus glucose metabolism in the muscle cell. Thus, regular resistance exercise training has been shown to increase 'insulin sensitivity', meaning the body can intake and utilize glucose more effectively (Pollock et al., 2000).Practical Application: Energy ProductionDuring resistance exercise there is a cascade of events that leads to an increase in several hormones that very specifically help deliver needed glucose for energy production to the working muscle cells.

Hormone Function in Resistance Exercise: Volume of TrainingIn resistance exercise, total volume is easily calculated by the number of reps x sets x weight that is performed in either a single session of resistance exercise or during a long-term resistance training program. Marx et al. (2001) examined the long term training (6-month training regime) adaptations associated with a low-volume (circuit) resistance training program versus a periodized high-volume resistance program in college-aged women. The study showed that the periodized higher volume resistance program had higher testosterone, insulin-like growth factor-1, and decreased levels of cortisol after the 24 weeks of training when compared to the circuit program. Greater increases in muscular strength, power, and speed were also seen in the high-volume group.
Smilios et al. (2003) examined the acute effects of the number of sets on testosterone, cortisol, and growth hormone responses after maximum strength (5 reps at 88% of 1RM, 3-min rest) and muscular hypertrophy (10 reps at 75% of 1RM, 3-min rest) protocols with 2, 4 and 6 sets of each exercise in 11 physically active (2-8 years resistance training experience) young men. Subjects also did a strength endurance (15 reps at 60% of 1RM, 1-min rest) protocol with 2 and 4 sets. In the muscular strength protocol, the number of sets did not affect the hormonal profile. In the muscular hypertrophy and strength endurance protocol, there was an increase in cortisol and growth hormone levels in four sets of exercise versus two sets. In this study, there was no significant increase in testosterone in any of the testing conditions. Contrariwise, Kramer and Ratamess (2005) summarize that protocols high in volume do tend to produce acute hormonal elevations in testosterone, as well as cortisol, and growth hormone.Practical Application: Volume of TrainingAcute and chronic research shows that higher-volume resistance programs tend to elicit the greatest hormonal responses.

Hormone Function in Resistance Exercise: Training to Failure versus Non-FailureIn a unique recent study, Izquierdo et al. (2006) examined hormonal responses in an 11-week resistance training to failure (one group) vs. non-failure (second group) followed by an identical (both groups) 5-week peaking period of maximal strength and power protocol. Subjects were 42 physically active males randomly assigned to the two groups. The results showed that 11 weeks of training to failure and not-to-failure resulted in similar gains in 1RM strength, muscle power output of the arm and leg extensor muscles, and maximal number of repetitions in the squat. However, after the identical 5-week peaking period of maximal strength and power training, the non-failure group showed greater increases in strength, power, resting testosterone levels, and reduced coritsol levels when compared to the failure group. The failure group did have a greater increase in muscular endurance in bench press repetitions and a decrease in insulin-like growth factor 1 (IGF-1), a muscle building hormone.Practical Application: Training to Failure versus Non-FailureTaking each set to failure may not be as an important factor as once felt when trying to increase muscular strength, power, and hormonal response for clients. By taking each set to failure, a trainer may actually make clients more susceptible to overtraining and decreased hormonal and muscle power adaptations.

Hormone Function in Resistance Exercise: Rest PeriodIn a recent study, Ahtiainen et al. (2005) examined a shorter rest period (2 minutes) in comparison to a longer rest period (5 minutes) in a 6-month long strength training protocol (2 heavy resistance training loading sessions per week for the lower body) with 13 recreationally trained men. Workout volume (reps x sets x weight) was equated for the two groups. It was shown that there were no differences between strength, mass, or hormonal profile (testosterone, cortisol, and growth hormone) in either the short versus the long rest periods in this 24-week study.Practical Application: Rest PeriodFrom a time perspective, trainers are always working to create the most time-efficient workouts for their busy clients. Previous research (Kraemer et al. 1990) suggested that a shorter rest period (one minute versus three minutes) elicited slightly higher acute hormonal responses; however, this newer 6-month study suggests no significant hormonal difference between the 2-minute versus 5-minute rest period on strength, mass, and hormone elevations.

Hormone Function in Resistance Exercise: Concentric versus Eccentric TrainingDuring conventional resistance exercise, there is a sequential concentric and eccentric muscle action. In training adaptations and hormonal responses, concentric muscle actions produced a greater amount of growth hormone when compared to an eccentric muscle action (Durand et al., 2003). Durand and colleagues compared both the concentric and eccentric muscle actions with the same absolute load. However, when compared using a relative load, both concentric and eccentric muscle actions produced similar growth hormone and testosterone responses (Kraemer et al., 2006).Practical Application: Concentric versus Eccentric TrainingFrom the hormonal response perspective, trainers are encouraged to vary resistance training schemes to incorporate and emphasize concentric and eccentric training protocols.

Hormone Function in Resistance Exercise: Forced versus Maximum RepetitionsForced repetitions are a popular method for adding intensity to any resistance exercise program. To perform a forced repetition requires the person to perform repetitions after the person has gone to failure. This type of training requires the assistance of a trainer (or workout partner). Maximum repetitions is synonymous with training to failure. Ahtiainen et al. (2004) investigated the hormonal responses of forced repetitions versus maximal repetitions in experienced strength athletes compared to nonathletes. Although the hormonal (testosterone, growth hormone, cortisol) increased similarly with both training loads (in both groups), the testosterone increases in the experiences weightlifters were significantly greater as compared to the maximal repetitions protocol.Practical Application: Forces versus Maximum RepetitionsTraining clients to take sets to and beyond failure should be used in moderation. As previously stated, always taking each set to failure can have negative effects on strength, power, and hormone responses (Izquierdo et al., 2006). Then again, it appears that the more trained the client, the greater the muscle developing hormonal response when forced repetitions are periodically incorporated.

Hormone Function in Resistance Exercise: Final ConsiderationsResistance exercise has been shown to dramatically affect acute hormonal responses in the body after training. These responses play a huge role not only in immediate tissue remodeling and growth, but as well as to long term strength, power, and hypertrophy gains. Resistance exercise protocols that stress large muscle mass (multi-joint exercises), are high in volume, and moderate to high intensity, tend to produce the greatest hormonal elevations for optimal muscular fitness benefits (Kraemer et al., 2005).

Bios:Luis M. Alvidrez and is currently in the master's program in exercise science at the University of New Mexico (UNMA). Luis owns a personal training company called Upward Motion Personal Training which specializes in corrective exercise and lifestyle management. His research interests are functional training, hormonal responses to exercise, exercise physiology, and lifestyle management.

Len Kravitz, Ph.D., is the Program Coordinator of Exercise Science and Researcher at the University of New Mexico where he recently won the "Outstanding Teacher of the Year" award. Len was honored with the 1999 Canadian Fitness Professional International Presenter of the Year and the 2006 Canadian Fitness Professional Specialty Presenter of the Year awards and chosen as the American Council on Exercise 2006 "Fitness Educator of the Year.